In the art of resistance heaters, there are a considerable number of forms of resistance heaters which are established of laminates of conductive and dielectric materials bonded together by suitable cements.
The most common and well-known form of laminate heater of the general character referred to above are flexible or semi-flexible blanket-type heaters such as are used in combination with and between water bed frame platforms and flotation mattresses to temper or control the temperature of the water in the mattresses.
The ordinary blanket-type heater is a thin, flat, horizontal unit of rectangular or other desired plan configuration and includes a central elongate heater element of a suitable metal arranged in a zig-zag, serpentine or other suitable pattern throughout the major portion of the plane of the heater; thin, flat, top and bottom laminates of a suitable dielectric material, such as flexible polyvinylchloride sheet stock; and a flexible heat resistance cement about the element and between the laminates, securely bonding the several parts together in tight, sealed relationship with each other. In addition to the above, such heaters include electric coupling means to connect ends of the heater elements with the conductors of power supply cables extending from the heaters to suitable power sources.
The above noted type of heaters are most often operated on or powered with commercial alternating current.
The above noted type of heaters are intended to heat a related piece of work and are commonly arranged with the outer surface of one laminate in substantially uniform heat conducting contact with an opposing surface of the work. Still further, it is not uncommon that the heaters are held in contact with their related work by supporting or backup structures which establish substantial uniform heat conducting contact with the outer surfaces of the other laminates. When heaters of the character referred to above are arranged in contact with their related work and/or supporting structures and are energized with alternating current, the resulting combination and relationship of parts establish capacitor structures which function to cause notable current leakage.
For example, in the case of the typical blanket-type heater noted above, when the heater is arranged with its top and bottom surfaces in substantial uniform contact with and between opposing surfaces of a piece of related work and with a supporting structure, the heater element and the work establish the equivalents of the plates and the top laminate establishes the dielectric of a capacitor structure, while the heat element and the supporting structure establish the equivalent of the plates and the bottom laminate establishes the dielectric of a second capacitor structure. One or the other, or both, of the above noted capacitor structures induces and/or results in current leakage from the heater to the work and/or to supporting structure. The current leakage which occurs can vary widely and is dependent upon dielectric constant of the materials of which the laminates and cement are established, the thickness of those materials and resulting spacing of the capacitor plate-like parts of the construction and the conductivity of the work and/or the supporting structures.
In the case of the ordinary water bed which includes a lower bed frame with mattress supporting wood platform having low electric conductivity, an upper flotation mattress filled with a large volume of mineral-ladened water having high electric conductivity and between which the heater is arranged, the capacitor structure established by the capacitor plate-like mattress and heating element and the capacitor dielectric-like top laminate has far greater capacitance and results in far greater current leakage than does the capacitor structure established by the capacitor plate-like platform and heating element and the capacitor dielectric-like bottom laminate.
In order to limit and/or better control current leakage in blanket type heaters of the character referred to above, principal attention had been given to the dielectric constant of the materials used to establish the laminates and the thickness of those laminates. Notably less attention has been given to the cements that are used to bond the parts of such heaters together. As a rule, the cements used are selected from those commercially available cements which have suitable bonding, curing, heat resistive and elastic characteristics to meet the manufacturer's requirements and the cost of which is minimal.
In practice, the above noted cements are really fair conductors of electricity and the effects of their conductive characteristics are reduced to a minimum by using the cements sparingly; that is, by applying them in very thin, sparsely distributed layers or coats.
It has been determined that the dielectric constants of commercially available cements suitable for bonding the laminates or laminated blanket-type heaters together are oftentimes excessively high and that the fluid and/or plastic nature of those cements is such that the thickness and uniform or even distribution of those cements, between adjacent laminates of heater structures, cannot be effectively controlled. As a result of the foregoing, the capacitance of the capacitor structures defined by the heaters, when in use, is not uniform or predeterminable within desirable limits.
Tests conducted with heaters of the character referred to above show that most heaters have spots or portions where the cement is thick and the heaters have what can be termed low capacitance where little current leakage occurs; and have spots and/or portions where the cement has been substantially displaced during manufacture of the heaters and where capacitance and resulting current leakage is relatively high and oftentimes excessive.
As a result of the foregoing, I have determined that there existed a great need for a way and/or means whereby the electric conductivity and/or dielectric constant of the cements employed in the manufacture of laminated blanket-type heaters and similar electrical structures can be notably reduced; and a great need for a way and/or means whereby the thickness of such cements and the resulting spacing of parts bonded together thereby can be effectively controlled, whereby, for example, the capacitance of those structures can be made uniform and within predetermined, narrow limits.